"Who" refers to the subject of the sentence. The subject of the sentence is the thing which is doing something.

"Whom" refers to the object of the sentence. The object of the sentence is the thing to which something is being done.

E.g. in "MLGlemon posts dank memes", 'MLGlemon' is the subject because you're doing the posting, and 'dank memes' is the object because the memes are being posted. You would say "MLG, who posts dank memes", or, "MLG browses hearthpwn with other users, for whom he posts dank memes" - although here there is now a second object of the sentence: other hearthstone users.

In this card, the text refers to Cave Hydra, who attacks another minion. Cave Hydra does the attacking, so it is the subject. 'Another minion' is the one being attacked, so it is the object. Cave Hydra attacks the minions next to 'the object', and so: "Cave Hydra also damages whomever is next to whomever it attacks". There are two 'whomever's here because there are two objects being referred to. The first 'whomever' refers to the minions next to the minion being attacked - one object. The second 'whomever' refers to the minion being attacked - another object.

Good for midrange and also build-a-beast combos. At the very least, it's a soft-taunt against agro on turn 3 as they will want to kill it before it attacks with a Houndmaster buff or an adapt from Crackling Razormaw. It has the right stats for a 3 mana card, having 4 health means it won't die to your typical turn 2, 3 attack minion.

I like how this card is fine on its own but has synergy with cards like Smuggler's Crate from Gadgetzan. That's how previous-expansion-support should work: through cards that work fine on their own but just so happen to synergize with previous expansions.

This card will not be played. The 3 mana slot for hunter is more crowded than a football stadium. It is not a big enough threat by itself and hunter does not have any useful buffs outside of the dream Houndmastercombo. It is useless on the defensive because you have to instigate the attack. Wont see play in constructed. If it had a 2 mana variant with less stats it would be great.

it's 4 health which is pretty good on turn 3 and it acts like a soft taunt because your opponent will be afraid of the houndmaster combo. The 3 slot is pretty over-saturated in hunter but you could remove Bearshark or Rat Pack to fit it in.